The overall aim of this renewal proposal is to uncover the mechanisms that underlie plasma volume expansion. They have developed two models to study these mechanisms. The human model, involving 32 min of intense exercise, produces a 10 percent plasma volume and albumin expansion within 24 hours. Their animal model, involving plasmapheresis, produces a complete recovery of both volume and albumin within 24 hours. The human and animal models differ from each other in certain respects but the two are sufficiently analogous to permit them to generate and test hypotheses in humans and then perform more invasive experiments to uncover mechanisms in rats. Using the human model they will test the following hypotheses related to the role of albumin synthetic rate, lymphatic protein transport and renal sodium handling in the process of plasma volume expansion: i) Sodium and water retention following saline loading is enhanced 24 hours following intense exercise. ii) Increased sodium retention following intense exercise occurs within both the proximal and distal tubules. iii) Enhanced sodium and water retention following intense exercise is mediated by changes in renal hemodynamics. iv) The elevation in plasma albumin content within the first two hours following intense exercise is mediated by increased lymphatic flow and thus modulated by changes in lymphatic outflow pressure. v) Albumin synthetic rate following intense exercise is modulated by posture dependent changes in circulating stress hormones. Using the animal model, they will test the following hypothesis: i) Increased hepatic albumin mRNA following chronic plasmapheresis is mediated by increased expression of hepatic nuclear transcription factor 1. ii) The primary mechanism for increased albumin content following plasmapheresis is lymphatic delivery of albumin to the vascular space. These studies characterizing the mechanisms underlying increased albumin synthesis and increased renal sodium reabsorption following exercise in humans and plasmapheresis in in vivo animal models will provide insight into the mechanisms underlying volume restoration. Such knowledge will lead to the development of methods to volume-expand compromised individuals more effectively without transfusion. Employment of both human and animal models enables the testing of hypotheses that are both relevant and mechanistic.